Within the scope of treating psoriasis, it is known to create what is known as a Psoriasis Area and Severity Index (PASI) and to document the course of the disease and the course of treatment using said index. So far, PASI has been determined manually or as a rough calculation or in the course of an estimate following examination of the skin surface of a patient. However, this determination has the disadvantage that it can only be carried out by highly specialized, medically trained and experienced professionals. Furthermore, determining PASI is highly time-consuming due to the time required for examining the entire skin surface. Finally, the most serious disadvantage in the previous determination of PASI is the fact that while there are basic criteria and an attempt to standardize PASI, they are applied differently by each user and even from patient to patient or even from examination to examination. As a consequence, the comparability of the determined values, the reproducibility of the determined values and, correspondingly, the entire informative value of the determined values of PASI in general decrease significantly.